The present invention relates to a method for controlling the beam formation of a downlink signal which is radiated by base stations of a first and a second radio cell to a mobile station in a mobile radio communication system. The present invention also relates to a base station suitable for implementing such a method.
In mobile radio communication systems such as the UMTS system, for example, base stations having a number of antennas are employed to utilize space diversity. These antennas are weighted using different weighting factors for data transmission to a mobile station; i.e., a downlink signal to be transmitted to the mobile station is applied to the latter in each case multiplied by the weighting factor assigned to the relevant antenna. The weighting factors are generally complex numbers which include an absolute and a phase component. A radiation lobe in the direction of the location of the relevant mobile station is thereby produced at the base station for each mobile radio station in a cell (beam formation). The weighting factors of the individual antennas are combined to form a weighting vector.
In so-called closed-loop transmit diversity schemes, the required weighting vector is estimated at the mobile station, quantized and transmitted via the uplink dedicated physical control channel to the base station where it is used for beam formation.
If the mobile station moves from a first radio cell of the mobile radio communication system to a second, a communication connection supported by it must be switched from the base station of the first radio cell to that of the second. This process is known as handover.
In the case of so-called soft handover, there is an intermediate state in which identical user data is transmitted in the downlink to the mobile station from the base stations of two or more radio cells. In industry literature, a distinction is drawn between a soft handover in the narrower sense whereby the radio cells each correspond to different base stations, and a softer handover whereby the radio cells correspond to different sectors of a base station. Where soft handover is referred to in the following, both alternatives will always be considered included.
Normally, in the case of soft handover of a mobile station between two base stations employing space diversity, a common weighting vector is selected for both base stations. This common weighting vector is determined in such a way that it maximizes the incoming power, at the mobile station, of all the radio cells involved in the soft handover; i.e., a weighting vector W is sought for which the expressionP=WH(H1HH1+H2HH2+ . . . )W is maximized, where H1HHi is the covariance matrix of the transmission channel of the ith base station involved in handover, to the mobile station. Consequently, all the base stations involved in the handover use the same weighting vector; i.e., they have the same spatial radiation characteristic.
FIG. 1 illustrates the above situation taking the example of the radiation lobes of two base stations BS1, BS2 and a mobile station MS located in the boundary region between the radio cells of the two base stations. As the two base stations BS1, BS2 apply the same weighting vector, their radiation lobes are identically oriented, and the mobile station MS is, in each case, located at the edge of the two lobes and therefore does not have optimum reception of the two base stations.
In order to increase the number of mobile stations that can be simultaneously supplied in a radio cell while at the same time minimizing interference in adjacent cells, it is intrinsically desirable to increase the number of antennas at the base station in order to be able to produce more directive radiation lobes. As FIG. 2 shows, this entails the risk of making a conventional soft handover impossible, as the radiation lobes of both antennas no longer overlap in the boundary region of the cells. The mobile station MS here receives none of the downlink signals of the base stations BS1, BS2 with sufficient quality.
An object of the present invention is therefore, to provide a method for controlling the beam formation of a downlink signal which enables downlink signals to be supplied to a mobile station which simultaneously communicates with base stations of at least two radio cells, the mobile station being simultaneously supplied with optimum quality from at least two of the base stations, and also to specify a base station suitable for implementing a method of this kind.